The present invention relates to lubrication systems for machinery.
Pressurized lubricating systems have long been used with machinery employing bearings and the like to reduce the friction and wear between machinery parts sliding on one another. Many of these systems are incapable of operating for even a relatively short period of time without the lubricant in a pressurized state. Particularly with large and/or expensive machinery, reserve lubricant supply systems have been employed which provide the required pressurized lubricant during shutdown of the equipment. Often, a reserve reservoir of oil is pressurized on a standby basis by a reserve pump, an elevated location for the lubricant reservoir or a pressurized body of compressed gas.
A compressed body of gas is a most popular form of pressurizing mechanism and employs a vessel to act as the reserve oil reservoir, a bladder within the vessel, and pressurized nitrogen or the like in the bladder. The vessel is in communication with the main oil supply and, thus, is held under the same pressure as the main oil supply during nominal operation. The amount of gas contained within the bladder is such that a pressure equilibrium between the inside of the bladder and the surrounding vessel is reached with the bladder substantially deflated. When the main oil supply pressure fails, the bladder then expands as the pressure drops, and forces the reserve oil from the reserve vessel.
The foregoing system is normally adequate for a great many uses where competing pressures do not exist within the equipment. However, with large centrifugal pumps, turbines, turbo-expanders and the like, the bearings supporting the shaft thereof are subjected to external pressures from either the impeller cavity itself or from a pressurized gas seal between the cavity and the bearing. Under such conditions, a relatively high pressure must be maintained throughout the operation of the reserve lubricant system. With the bladder type system, as the bladder expands the pressure within the bladder drops. For example, an oil supply may be maintained at 1,000 p.s.i. which is used to supply a system requiring 950 p.s.i. Upon interruption of the 1,000 p.s.i. pressure, the expansion of the gas within the bladder from 1,000 p.s.i. to 950 p.s.i. displaces an amount of oil which is equal to only 5% of the volume of the gas prior to expansion. If the vessel is divided equally between oil reservoir volume and bladder volume, only 2.5% of the vessel's volume of oil is available in the usable range. If the vessel has a total capacity of ten gallons, the usable supply of oil at or above 950 p.s.i. is only one quart of oil. In the type of equipment in which such systems are employed, a normal rate of oil usage may be twenty gallons per minute. The bearing would thus be supplied with emergency oil for less than one second. A more normal shutdown time for such equipment is on the order of five to thirty seconds. Thus, reserve systems as described above are inadequate.
Naturally, substantial increases in the stored pressurized gas can effectively overcome the problem of rapid pressure loss in such systems. One such proposal is disclosed in U.S. Pat. No. 4,002,224, issued to Easter. However, it is not always advantageous or practical to maintain such a large source of pressurized gas.